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fold, in geology, bent or deformed arrangement of stratified rocks. These rocks may be of sedimentary or volcanic origin. Although stratified rocks are normally deposited on the earth's surface in horizontal layers (see stratification ), they are often found inclined or curved upward or downward. Arches, or upfolds, in stratified rock are called anticlines; depressions or downfolds, synclines. A third type of fold, the monocline, is a steplike structure sloping in one direction only. It is more correctly called a flexure and generally passes at depth into a fracture called a fault. An imaginary line drawn along the crest of an anticline or the trough of a syncline is its axis; the two sides curving away from the axis are the limbs. If both limbs, dipping in opposite directions, make the same angle with the horizontal, and if an imaginary axial plane passed through the axis and the center of the fold is vertical and divides the fold into two equal halves, the fold is symmetrical; if the limbs make unequal angles, and if the axial plane is inclined and does not bisect the fold, the fold is asymmetric. If one limb lies partly under the other, and the axial plane is inclined, the fold is overturned; if one limb lies almost completely under the other, and the axial plane is almost horizontal, the fold is recumbent. The axis of a fold cannot be indefinitely extended parallel to the horizontal, but plunges or emerges as the fold tapers off to a plane. Certain domes are very short anticlines with axes plunging at both ends, while some basins, similarly, are synclinal structures. Folds are commonly formed at some distance below the surface, but complete folds or portions of folds are exposed by erosion. Anticlines frequently have their crests eroded, till only the worn-down stumps of the two limbs remain. In a similar manner synclines may be eroded so that only the edges of the limbs project above the surface. The ridge crests of the Appalachian Mts. are eroded limbs of folds. The nature of the original fold can generally be determined from the arrangement of the outcrops, or exposed portions; thus, two outcrops dipping toward each other mark a syncline, and two outcrops dipping away from each other, an anticline. Folds on a grand scale, extending, for example, most of the length of a continent, are known as geosynclines and geanticlines. The immediate cause of folding is generally conceded to be the horizontal compression of the earth's surface, anticlines being squeezed up by this compression and synclines formed between anticlines. The problem of the ultimate cause of fold formation is similar to that of fault formation, both being earth movements involved in mountain building and plate tectonics . Porous and permeable rocks of anticlines often contain oil and gas reservoirs. Organic remains of late Paleozoic tree fern swamps were converted to anthracite coal during the folding of the Appalachian Mts.